How to add a field to a matlab struct.

I have a matlab function that takes a struct array in my workspace as an input.The struct has fields a and b. I want to add another field c in the struct. Please how fdo I do this.

Answer 1

[madhan ravi]

yourstruct.c=...;
% doc setfield()

[Daniel Boateng]

function [b]= Testing2(old)

b= old.c;

end

I tried but i get this error

Reference to non-existent field 'c'.

Error in Testing2 (line 2)

b= old.c;

[madhan ravi]

old.c=...; the values that you want to assign to the field c

[Daniel Boateng]

Thanks for your comment. I tried it but in my output i still have the old subfield but not the new one. Is there something I am missing?

Thanks

function [b]= Testing2(old)
b= old;
old.c= 'blue';
end

[madhan ravi]

Yes, you didn't call the function:

new = Testing2(old) % function call

function new = Testing2(old) % function definition
new = old;
new.c = 'blue';
end

[Guillaume]

Is there something I am missing?

Yes, of course. Your code

function b = Testing2(old)  %note that old is a copy of the variable that was passed as input to the function
    b = old;    %so b is now a copy of old, completely independent of old
    old.c = 'blue';  %modify old. DOES NOT modify b   
end  %function ends, returns b, discards the modified old since it's not an output.

The fix is to modify b not old:

function b = Testing2(old)
    b = old;
    b.c = 'blue';
end

or even simpler

function old = Testing2(old)
    old.c = 'blue';
end

The latter code may be more efficient under some circumstances as well.

[Daniel Boateng]

Thanks bro. It works perfectly

[madhan ravi]

Thank you Guillaume for the explanation.